The present invention relates to a ball screw mechanism in which, even when a retaining piece is interposed between two mutually adjoining load balls, decrease in the number of load balls can be controlled as much as possible to thereby prevent the load balls from degrading the load capacity and rigidity thereof, can reduce friction between the load balls and retaining piece to thereby enhance the circulating performance of the retaining piece, and can prevent the load balls from butting against each other to thereby prevent degradation in the operation efficiency thereof, deterioration in the quality of sounds generated, and the friction and damage of the load balls.
Conventionally, in a ball screw mechanism, as shown in FIG. 9, on the outer peripheral surface of a screw shaft 1 and on the inner peripheral surface of a nut 2, there are respectively formed spiral-shaped screw grooves 3 and 4 which disposed opposed to each other and, in a screw groove circulation passage defined by the two screw grooves 3 and 4, there are disposed a large number of load balls 5 in such a manner that they are free to roll. In case where the screw shaft 1 and nut 2 are rotated with respect to each other to thereby move one of them in the axial direction thereof, the screw shaft 1 and nut 2 are smoothly moved in a spiral manner with respect to each other.
In such ball screw mechanism, the load balls 5 are arranged densely within the screw grooves 3 and 4 and they roll in the same direction within the individual screw grooves 3 and 4; and, in the rolling movements thereof, at contact points between the two mutually adjoining load balls 5 and 5, the load balls 5 rolling in the mutually opposite directions are contacted with each other to thereby interfere with their mutual rolling movements, with the result that the load balls 5 and 5 are caused to slip at the contact points. This interferes with the free rolling movements of the load balls 5, degrades the operation efficiency of the load balls 5, gives rise to friction and damage in the load balls 5, causes variations in the torque of the load balls 5, and swells the noises that are produced by the load balls 5.
To cope with these problems, as shown in FIG. 10, between the mutually adjoining load balls 5, there are respectively interposed spacer balls 6 each having a diameter smaller by several tens .μ. than the load balls 5. That is, such interposition of the spacer balls 6 prevents the load balls 5 against slippage, improves the operation efficiency of the load balls 5, and reduces the friction and damage of the load balls 5 to thereby prevent variations in the torque.
However, in the ball screw mechanism shown in FIG. 10, while the load balls 5 are, for example, ten in number, the spacer balls 6 are also, for example, ten in number. Therefore, when compared with the ball screw mechanism shown in FIG. 9, a clearance between the two adjoining load balls 5 is large and the load balls 5 are reduced in number down to about one half, which decreases the load capacity of the ball screw mechanism and also degrades the rigidity thereof.
In view of the above problems, there is also known a structure in which, as shown in FIG. 11, between every two mutually adjoining load balls 5, there is interposed a retaining piece 10 having two concave surfaces 11 respectively facing the two load balls 5. According to this structure, while the load balls 5 are in contact with the concave surfaces 11 of the retaining piece 10, they are allowed to circulate well within the spiral-shaped screw grooves 3 and 4. Therefore, when compared with the conventional structure using spacer balls, the spacer, that is, the retaining piece 10 can be made thinner, which makes it possible to control reduction in the number of load balls and thus avoid degradation in the load capacity and rigidity of the ball screw mechanism.
However, in the ball screw mechanism shown in FIG. 11, since the relation between the outside dimension of the retaining piece 10 and the outside dimension of the load ball 5 is not always considered properly, there is a fear that, when the retaining piece 10 passes through the spiral-shaped screw groove circulation passage and tube circulation passage, it can interfere with the inner walls of these circulation passages. Therefore, such interference makes it difficult for the load balls 5 to circulate stably, thereby giving rise to generation of variations in the torque, or causing the retaining piece 10 to wear.